1. Field of the Invention
The present invention is generally directed to the extracellular domain of p185.sup.HER2, a receptor-like protein which is encoded by the human homolog of the rat neu oncogene.
More specifically, the present invention is directed to a form of the extracellular domain which is essentially free of transmembrane and cytoplasmic domains, to the DNA encoding this form, and to a process for producing this form of the extracellular domain in a host cell.
2. Description of Background and Relevant Materials
Human epidermal growth factor receptor 2 (HER2, also known as NGL and human c-erbB-2, or ERBB2), is the human homolog of the rat proto-oncogene neu. HER2 encodes a 1,255 amino acid tyrosine kinase receptor-like glycoprotein with homology to the human epidermal growth factor receptor. Although no ligand binding to this probable growth factor receptor has yet been isolated, the HER2 gene product, p185.sup.HER2, has the structural and functional properties of subclass I growth factor receptors (Yarden et al., Ann. Rev. Biochem., 57:443-478 (1988); Yarden et al., Biochem., 27:3113-3119 (1988)).
The receptor tyrosine kinases all have the same general structural motif; an extracellular domain that binds ligand, and an intracellular tyrosine kinase domain that is necessary for signal transduction, or in aberrant cases, for transformation. These 2 domains are connected by a single stretch of approximately 20 mostly hydrophobic amino acids, called the transmembrane spanning sequence. This sequence is thought to play a role in transferring the signal generated by ligand binding from the outside of the cell to the inside. It has also been suggested to play a role in the proper positioning of the receptor in the plasma membrane.
Consistent with this general structure, the p185.sup.HER2 glycoprotein, which is located on the cell surface, may be divided into three principle portions: an extracellular domain, or ECD (also known as XCD); a transmembrane spanning sequence; and a cytoplasmic, intracellular tyrosine kinase domain. While it is presumed that the extracellular domain is a ligand receptor, as stated above the corresponding ligand has not yet been identified.
The HER2 gene is of particular interest because its amplification has been correlated with certain types of cancer. Amplification of the HER2 gene has been found in human salivary gland and gastric tumor-derived cell lines, gastric and colon adenocarcinomas, and mammary gland adenocarcinomas. Semba et al., Proc. Natl. Acad. Sci. USA, 82:6497-6501 (1985); Yokota et al., Oncogene, 2:283-287 (1988); Zhou et al., Cancer Res., 47:6123-6125 (1987); King et al., Science, 229:974-976 (1985); Kraus et al., EMBO J., 6:605-610 (1987); van de Vijver et al., Mol. Cell. Biol., 7:2019-2023 (1987); Yamamoto et al., Nature, 319:230-234 (1986).
Gene transfer experiments have shown that overexpression of HER2 will transform NIH 3T3 cells and also cause an increase in resistance to the toxic macrophage cytokine tumor necrosis factor. Hudziak et al., "Amplified Expression of the HER2/ERBB2 Oncogene Induces Resistance to Tumor Necrosis Factor Alpha in NIH 3T3 Cells", Proc. Natl. Acad. Sci. USA 85, 5102-5106 (1988).
Because amplification of the HER2 gene results in greatly increased numbers of the p185.sup.HER2 protein residing on the surfaces of affected cells, there may be an interrelationship between increased amounts of p185.sup.HER2 extracellular domain on the surfaces of affected cells and the resistance of these cells to treatment. It would therefore be highly desirable to be able to manipulate the p185.sup.HER2 extracellular domain in order to investigate several possibilities for the treatment of conditions associated with amplification of the HER2 gene. These therapeutic modes relate not only to the extracellular domain, but also to the putative ligand, which it should be possible to isolate and characterize using the purified p185.sup.HER2 extracellular domain.